This invention relates to nonmagnetic austenitic stainless steels which are balanced in composition to provide a 0.2% yield strength of at least 100 ksi (689 N/mm.sup.2) in the hot-worked or forged condition, improved resistance to intergranular attack and to the production of drill collars fabricated therefrom.
Directional drilling of oil wells requires a nonmagnetic drill collar to insure the proper functioning of the electronic measuring and guiding instruments. The greater drilling depths require steels which have improved strengths and also the capability to resist stress corrosion cracking and particularly intergranular stress corrosion cracking caused by high levels of chlorides in the drilling mud and sea water.
Intergranular stress corrosion cracking is believed to be caused by the depletion of chromium at the grain boundaries due to the formation of chromium carbides and nitrides and accelerated by the application of a tensile stress across the grain boundaries. If the carbon level is reduced to below about 0.03% or strong carbide forming elements, such as niobium or titanium, are added, the resistance to intergranular attack has been improved in many austenitic stainless steels. Other steels have higher chromium levels to allow for the depletion. This has been the typical approach to solve the problem.
Since precipitation of carbides and nitrides will depend not only on composition, but also time and temperature, the process of producing the drill collars will influence the properties of the finished collar.
Existing drill collar alloys fall into some general categories. One group will typically have about 13% chromium, 0.06% carbon, 17% manganese, 0.3% nitrogen, 2% nickel and 0.3% niobium. The other family of alloys will typically have about 17.5% chromium, 0.15% carbon, 11% to 17% manganese, 0.3% to 0.35% nitrogen, 0.5% to 6% nickel and no niobium.
The first group uses low chromium levels to simplify keeping the composition nonmagnetic. Lower levels of carbon and nitrogen are required and the addition of niobium provides the additional strength to compensate for the low carbon and nitrogen. High manganese levels are required for austenite stability.
The second group of alloys has higher chromium levels for corrosion resistance. This necessitated the higher levels of carbon and/or nitrogen to maintain an austenitic structure which is nonmagnetic. Since the carbon levels are high, the manganese range can be less restrictive for austenite stability. No niobium is used to add strength and stabilize the carbon. This eliminates another ferrite former from the system and relies upon the carbon and nitrogen for strength.
Both groups of alloys illustrate the importance of balancing the composition to be nonmagnetic and capable of generating high strength levels. However, both approaches will be inadequate in developing the level of resistance to intergranular stress corrosion cracking required for the environment of drill collars.
The present invention provides a composition balance and processing conditions to allow the production of drill collars having the combination of properties including strength, nonmagnetic stability, and resistance to intergranular stress corrosion.